1. Field of the Invention
The present invention relates to a process for cleaning a silicon substrate which is bonded by hydrogen ion implantation method, in particular a process for cleaning a silicon substrate, which is a raw material, before being bonded.
Priority is claimed on Japanese Patent Application No. 2005-121005, filed Apr. 19, 2005, the content of which is incorporated herein by reference.
2. Description of the Related Art
Silicon On Insulator(SOI) substrates, which are formed by bonding silicon substrates to each other in which hydrogen ions are implanted, is attracting attention as semiconductor substrates for use in next-generation very large-scale integrated circuits i.e. VLSI, because SOI substrate demonstrate various excellent characteristics, such as high speed, low power consumption, operability at high temperature, and radiation-resistant effect, in the case in which integrated circuits are formed using SOI substrates.
Hitherto, as the process for producing SOI substrates, a process is known, as shown in FIG. 4, which includes (a) forming an oxide film on a first silicon substrate surface, using a thermal oxidation method, etc., (c) implanting hydrogen ions in the surface of the substrate so as to form a fine bubble layer, i.e. an implanted layer, within the substrate, (d) cleaning the substrate in which hydrogen ions are implanted, (e) bonding the first silicon substrate to a second silicon substrate, (f) exfoliating the two substrates from each other while making the fine bubble layer be a divide line, (g) leveling the fine bubble layer on the surface of the first silicon substrate which exists on the exfoliated second silicon substrate, while intervening an oxide film therebetween, and (h) thinning the first silicon substrate into a predetermined thickness by processing such as grinding, as is disclosed in patent document 1 (i.e. International Publication No. 01-093334, pamphlet).
In the method disclosed in patent document 1, cleaning is performed after performing ion implantation in order to remove the particles on the silicon substrate. Moreover, as the cleaning method, cleaning (SC-1 cleaning) using a mixed solution containing NH4OH, H2O2, and H2O is disclosed.
Furthermore, as a method of performing the substrate-cleaning and the hydrogen ion implantation, dividing each of the substrate-cleaning and the hydrogen ion implantation into several steps, is also proposed, as is disclosed in patent document 2(i.e. Japanese Unexamined Patent Application, First Publication No. 2003-309101 official report). As the cleaning method, cleaning using ammonia, hydrogen peroxide, pure water, electrolysis ion water, a mixed-solution (SC-1) containing NH4OH, H2O2, and H2O, etc. is disclosed. In accordance with this method, it is possible to reduce the generation of a portion which is formed right under a particle and becomes a shadow of ion-implantation, thereby causing generation of voids, in addition, it is also possible to reduce the area in which the hydrogen ion concentration is significantly low so as to eliminate exfoliation fault after heat treatment, thereby preventing the formation of voids.
Although these ion implantation exfoliating methods are excellent as a method for producing a bonded SOI substrate, it is necessary to decrease miss-bonding, known as a void, which is generated at an interface of adhering two substrates, in order to produce these SOI substrates with high yield at a mass-production level. It has turned out that the particles adhered on the surface mainly cause voids, and it turns out that if the particle size of the particles is 0.5 μm or more, then voids will be generated.
In the case in which no particles are adhered when performing ion implantation, as shown in FIG. 5(a), a hydrogen ion-implanted layer 12, in which hydrogen ions are implanted uniformly throughout an area which becomes an active silicon layer 13 later, will be formed beneath an oxide film 11. If such a uniform hydrogen ion-implanted layer 12 is formed, then it is possible to exfoliate clearly, at an exfoliated surface S that is brittle, within the hydrogen ion-implanted layer 12, during the exfoliating step.
On the other hand, in the case in which particles are adhered when performing ion implantation, part of a particle P will not be adhered during an adhering step, as shown in FIG. 5(b), in addition, the portion under the particle P becomes shadow of the particle P so that a predetermined quantity of hydrogen ions will not be implanted thereto, thereby generating a hydrogen ion depletion area 14. If the hydrogen ion depletion area 14 is generated, then this portion does not become brittle, and as a result, no continuous exfoliated surface S will be obtained during the exfoliating step, thereby it becomes impossible to exfoliate clearly.
Therefore, in order to reduce such a void, it is necessary to perform substrate cleaning to completely remove the particles adhered to the silicon substrate surface on which ion implantation will be performed, before performing ion implantation.
If hydrogen ion implantation is performed in steps and cleaning between ion implantation is performed, then it is possible to remove particles to a certain degree, however, most of the particles which entered before performing ion implantation will be fixed to the surface of the substrate by performing ion implantation, so that it becomes impossible to remove the particles by cleaning with a chemical fluid. Moreover, if cleaning between ion implantation is performed, then the surface of the silicon substrate on which ion implantation is performed becomes coarser than that of an ordinary substrate due to the cleaning, and since cleaning is repeated plural times, roughening of the silicon substrate surface will progress, thereby generating voids etc., when bonding the substrates. In addition, if ion implantation is performed in steps while inserting cleaning steps therebetween, then the number of steps during production increases, thereby making it complex, and as a result, productivity will deteriorate remarkably.
Thus, it is an object of the present invention to eliminate particles on the surface of the substrate, which cause a shadow when performing ion implantation and voids when bonding substrates with each other, and provide an effective cleaning method without increasing the number of production steps.